Silenence in the library odd image ?

Watching 'Silence in the library' Chapter 4 track 14 at time 15:11 to 15:12 there is an odd image ? that passes in back of the Doctor and Donna very quickly, then fades away, can't make out what it is, has anyone else seen this and can tell what it is. ?

It's Miss Evangelista. You can clearly see her hair scrunchy; and there's no one else in the scene with hair of that color or shape. Why does she cross behind the Doctor and Donna? I dunno. In the wideshots before and after the cross she's on the same side of the Doctor (left). So the continuity-minder kinda messed up here. But it's definitely Evangelista moving from the left of the Doctor to the right of Donna.